1. Field of the Invention
The present invention relates to an analog-to-digital converter (ADC). More particularly, the present invention relates to switched capacitor ADC with programmable gain control.
2. The Prior Art
Many techniques are known in the prior art for analog-to-digital (A/D) conversion. Each of these A/D techniques has advantages which correspond to the application in which the A/D conversion is being performed. Choosing the A/D conversion technique to be used in a particular application can depend on the consideration of at least the speed, accuracy, cost, dynamic range and power requirements of the application. The spectrum of A/D conversion techniques available in the prior art generally fall into one of two categories.
In the first category are A/D techniques where the analog input signal is directly compared to a digital reference value. The digital value output from the A/D conversion is equal to the digital reference value which most closely compares to the analog input signal. This category of A/D converters is considered fast, however, to obtain high resolution with A/D techniques in this category is generally expensive. A/D techniques for direct comparison include, for example, parallel encoders and the successive approximation method.
In a parallel encoder, there are a plurality of comparators, each of which has the analog input signal connected to a first input. Comparison of the analog input voltage is made to a reference voltage connected to a second input of each of the comparators. The reference voltages supplied to each of the comparators all have different values. The values of the reference voltages are generally evenly spaced. The reference voltage corresponding most closely to the analog input signal as determined by a priority encoder constitutes the digital output of the A/D converter.
The successive approximation method is essentially a binary search performed by comparing the output of a D/A converter with the analog input signal. The value of the output of the D/A converter is usually set by a most significant bit input to the digital side of the D/A converter. The digital inputs to the D/A converter are changed until the analog output value of the D/A converter matches the value of the analog input signal. In a variation of this method, an up/down counter is used to generate the reference values presented to the digital side of the D/A converter. As the analog input changes, the up/down counter changes the reference values presented to the D/A converter to track the analog input voltage.
In the second category are techniques where the analog input is converted into a second quantity which is then used to represent the digital value corresponding to the analog input signal. The second quantity may be, for example, a pulse train at a frequency which varies to represent the value of the analog signal or a specific count generated for a specified time period, wherein the count is proportional to the analog value of the input signal during the specified time period. These techniques include voltage-to-frequency conversion, single-slope integration, dual-slope integration, and delta-sigma modulation.
In a voltage-to-frequency technique, the analog input controls the frequency of a variable frequency oscillator. The oscillator output charges a capacitor which is compared with the analog input signal. The oscillator frequency is varied until the input levels are the same. The frequency of the oscillator is proportional to the analog input signal.
In a single-slope integration, a ramp voltage is generated, usually by storing charge in a capacitor. During the time period the charge is being stored on the capacitor for comparison with the analog input signal, a counter is operating. When the ramp voltage equals the analog input voltage, the counter is stopped. The value in the counter is proportional to the analog input voltage. In a dual-slope integration, during a first fixed time period a current proportional to the analog input signal charges a capacitor. The charge on the capacitor is then discharged to zero at a constant rate. The length of time taken to discharge the capacitor to zero is proportional to the analog input.
In an A/D converter using prior art delta-sigma modulation techniques, an analog input signal is oversampled and fed into a differential amplifier which operates as a summing junction. The differential amplifier sums the analog input with a feedback signal that is the output of the delta-sigma modulator. The output of the differential amplifier thereby represents the change in the value of the analog input signal from one sample to the next.
By feeding back the output of the delta-sigma modulator into the differential amplifier, the output of the differential amplifier is kept at a zero average signal value. Because of this feature, a delta-sigma modulator is alternatively known in the art as a charge-balancing modulator. The successive outputs of the differential amplifier, which in summation represent a zero average signal value, are fed into an integrator for summation. The output of the integrator is fed into a comparator for comparison with a reference value. The comparators employed in the prior art require both positive and negative power supplies and a very accurate midpoint reference such as ground.
When the integrated value is above the reference value, the output of the delta-sigma modulator is a high value, and a high value is fed back to the differential amplifier. When the integrated value is below the reference value, the output of the delta-sigma modulator is a low value, and a low value is fed back to the differential amplifier. However, because the value of the signal being fed back is either a constant high or low value, the output of the differential amplifier cannot be a full rail-to-rail swing.
The high and low signals are treated as up/down signals which are filtered by a digital filter. The digital filter may be, for example, an up/down counter which accumulates the up/down signals. After processing a selected number of analog cycles, the output of the digital filter is used to determine the digital output of the A/D converter. The digital output is the average value of the analog input signal during the time represented by the selected number of analog samples.
One of the major advantages associated with delta-sigma modulators is that low resolution components can be used to process the analog input signal, and a high resolution digital output can be extracted because the analog input signal is oversampled. However, with the ever increasing use of digital signal processing in many different applications, there exists in delta-sigma modulators a need for greater design simplicity, and lower power requirements.